1. Field of the Invention
The present invention generally relates to plastic materials and articles of manufacture and, more particularly, to a method for providing relief for interference press-fit type deformation and an exemplary device incorporating such plastic deformation relief.
2. Description of the Related Art
Molding technology advances in the field of materials having plastic properties have resulted in many articles of manufacture which now unite components by press-fitting them together when one or more of the components is fabricated of such a plastic material. By definition, stress and strain forces exerted on the material during such press-fitting cause deformation of the plastic material which thereby allows a relatively tight connection.
For example, advances in molding technologies have allowed molded gears, e.g. of hard plastic, to replace machined gears and, thereby, substantially reduce product costs. As other examples, irregulars cams, normally shaft-mounted disks (such as flywheels, disk brake rotors, and clutch plates), and even popular children's construct-it-yourself toys use press-fit parts. For the purpose of describing the inventive concept disclosed, an example of a spur gear having integral deformation relief features will be used in this specification. This is not intended by the inventors to express or imply any limitation on the concept.
It is well-known that hobbed metal gears can be mounted on a shaft by simple pressing. Machining such gears and their respective shafts allows very tight tolerances to be achieved. The gear mount hole may or may not have features, such as knurls, to mate with complementary shaft features in order to prevent slippage.
A problem with press-fit plastic gears is how to mount the gear on a shaft in such a way as not to disturb the concentricity of the gear to the shaft rotational axis and yet achieve a tight tolerance in order to prevent slippage. An eccentricity will cause angular transmission error--which is important if positional accuracy is required. Furthermore, an eccentric gear of a plastic material may not transmit torque properly and may quickly be worn out.
In the prior art, it is generally known to have a gear mount hole which is D-shaped with a close tolerance to the cross-section dimensions of a D-shaped shaft. The gear is then often secured with E-rings or screws. The problem with such a mounting technique is that close tolerances available in the machining of metal gears are difficult to achieve with current molding technologies. If the gear mount hole is slightly smaller than the shaft, some deformation will still exist once the gear is pressed onto the shaft. Holding screws for oversized shaft holes can also create unwanted deformations and resultant eccentricities.
Hence, there is a need for a method of relieving deformation in interference press-fit parts.
With respect to the exemplary embodiment, there is a need for an economical, plastic-material gear which can be mounted on a shaft in such a way as not to disturb the concentricity of the gear to the shaft rotational axis and yet still be able to accurately position and transmit the torque required.